Organic transistor means, in the invention, any device comprising three electrodes (source, drain, gate), and a layer of a dielectric compound separating the gate from a layer of an organic semiconductor material. Organic semiconductor means, in the invention, a compound whose mass is at least 50% based on organic compounds.
A crucial point for making electronic circuits based on organic transistors is control of the threshold voltage of the transistors, i.e. the gate voltage to be applied for generating the transition between the blocked behavior of the transistor and its conducting behavior.
In fact, the threshold voltage of the organic transistor is a key element for making circuits on an industrial scale.
The deposition of layers of organic semiconductors on surfaces having different surface energies is in itself a significant problem. This is all the more true of the crystalline or semicrystalline organic semiconductor materials, for which crystallization is an essential factor for obtaining good performance. Kobayashi et al. in Nature Materials, Vol. 3, May 2004 p 317-322 and Wu et al. in APPLIED PHYSICS LETTERS, 86, 142102 (2005) demonstrated that the use of molecular layers can make it possible to alter the threshold voltage of organic transistors based on amorphous, crystalline or semicrystalline semiconductors. This change in threshold voltage is mainly attributed to the existence of surface charges at the interface of the dielectric material/organic semiconductor material.
Nevertheless, for crystalline or semicrystalline materials, an appreciable change in transistor performance is observed, in addition to the change in threshold voltage. For example, the mobility of the charge carriers varies very significantly. This is attributed to the fact that crystallization takes place differently depending on the surface energies, which vary in relation to the monomolecular layers used. In fact the molecules making up the molecular layers comprise end units which vary, and which therefore induce a different initiation of crystallization.